The source of my confusion becomes clear! I don't recall ever seeing
a header file 'include' a source file before, so I didn't even look at
the code. <hangs head in shame>

Is this practice specific to c++ ? (If you know of any examples of c
code doing this, please point them out. Thanks!)

This is not specific to c++, its more specific of aplications that
have the source code badly organized, and with twised file dependency
chains.
So, it's possible to have "bad" organization like that in C too.